1. Field of the Invention
The present invention relates to a filter for removing soot particles from waste gases using a filter body of porous material traversed by filter passages arranged in honeycomb form, and in the zone of the entry openings of the filter passages that open on the gas entry side of the filter there are arranged at least two resistance heating elements forming heating zones for heating the adjoining zones of the face surface of the filter body.
These resistance heating elements are connected from time to time individually or in common with a current source, in order to, in cooperation with gases flowing through the filter body, burn off soot particles accumulated on the walls of the passages.
2. Description of the related art including information disclosed under 37 CFR .sctn..sctn. 1.97-1.99
A filter similar to the filter disclosed herein is known from W. German published patent specification DE-OS 37 12 333.In this patent publication the resistance heating elements forming the individual heating zones are wires laid in a harp pattern or a meandering pattern on the face surface of the filter body, which form wire loops plunging over or projecting into the entry openings of the filter passages into filter passages. By its nature, the cross section of a wire is constant over its length, whereby the same amount of heat is liberated over its entire length. Thereby the wire heats up also where it should not or must not.
Furthermore, the length and cross section of the wire must be attuned to one another for the achieving the ignition temperature, and, as a result, there is no freedom in design of the disposition and dimensioning of the heating elements and the heating zones. Additionally, the laying of the thin wires with their loops on the face surface of the filter body as well as their connecting with the outward-leading current lines in the fabrication of the resistance heating element is an extremely difficult and time-consuming operation.
As will be described in greater detail hereafter, it is the aim of the present invention to create resistance heating elements which allow freedom of design in the heating zone which are simultaneously inexpensive to manufacture and to install, and are dependable and long-lived.